The invention relates generally to current transient suppression circuitry and more particularly to such circuitry for controlling and limiting inrush current transients associated with turn-on of power supplying circuits.
Current transients in rectifier and similar such power supplying circuits, for example, are generally characteristic of the charging of uncharged capacitive components such as capacitive loads upon the initial application of a voltage to the circuit. This initial charging period of substantially uncharged circuit capacitance will hereinafter be referred to as circuit turn-on. These transients are particularly attributable to the charging of large value filter and storage capacitors which are typically connected between the output terminals of a rectifier to filter the rectified unregulated voltage. At turn-on of power supplying circuits, the voltage between the output terminals attempts to increase in a step-wise manner, however, the uncharged capacitors appear as the electrical equivalent of a short circuit, resulting in a large initial surge current.
In a rectifier circuit having a sinusoidally varying input voltage, this surge current will be a maximum when the circuit is switched on at a point in time when the input voltage is at or near its peak value. Though brief in duration, this initial current surge or inrush current may be orders of magnitude greater than the normal operating current. The problem is compounded when, in addition to the capacitance internal to the rectifier circuit itself, the circuitry to which power will be supplied is also capacitative and substantially uncharged, adding to the total circuit capacitance. This additional parallel capacitance adds to the value of internal capacitance, whereupon applying a turn-on voltage V.sub.i, the surge current I.sub.i may be expressed as I.sub.i =Cdv/dt where C is the total circuit capacitance and dv is the instantaneous change in voltage over a minutely short period of time. Evidently, increasing C and having a relatively small dt increases I.sub.i. Circuitry experiencing the surge current must be designed to accomodate the momentary higher current or alternatively, means must be provided to suppress this initial current transient.
Conventionally, relatively large values of series inductance (series with respect to the internal capacitance and circuit load) provide the requisite impedance to suppress turn-on current transients. An inductance appears as an open circuit to an instantaneous change in voltage, thereby presenting a large impedance to current transients caused by the initial application of input voltage to the circuit.
Conventionally, an L-C filter network serves both to filter unwanted A-C ripple, and to impede inrush current resulting from rapid voltage changes. However, the convention of using series inductance to suppress inrush current transients suffer several drawbacks, especially in relatively high power applications. To be of sufficient impedance to suppress inrush current in typical rectifier circuits, the inductor must be of a relatively large value. A large inductance in series with the full wave rectifier may extend each rectifier's conduction period such that conduction in one rectifier does not terminate until the other rectifier starts conducting, resulting in a reduced average output voltage and current. Additionally, a large value of inductance is conventionally obtained using an iron core inductor which is physically large, heavy and expensive relative to other component costs. Such an inductor will be shunted with a large amount of stray capacitance and the nonlinear properties of the iron core may cause undesirable signal distortion. Furthermore, the large time constant produced by using the large inductance in series with the large capacitance may not be desirable subsequent to turn-on. Therefore, it is advantageous to provide current transient suppression only for the duration of "turn-on" or initial current surge, and thereafter remove or disable the suppression circuit from the rest of the circuitry for circuit operation subsequent to turn-on (i.e., when all capacitive components are substantially charged).